Power shift transmissions are known in the earthmoving and trucking vehicle industries which offer a large number of gear ratios by utilizing a plurality of serially interconnected planetary gear sets and a plurality of clutches and brakes for effecting the individual gear steps. U.S. Pat. No. 4,205,563 issued to J. M. Gorrell on June 3, 1980 is generally representative of such planetary transmissions. In that embodiment a range planetary gear unit and a reversing planetary gear unit are coupled together at the output end of the transmission and are serially connected to a speed planetary gear unit at the front or input end to provide seven forward and one reverse speed ratios. The output shaft is connected to the planet carrier of the range planetary gear unit through a centershaft which must carry relatively high torque so that the centershaft must be of substantial diameter and connected by strong joints to the associated elements. Since the centershaft extends forwardly within the rear reversing planetary gear unit, the reversing planetary gear unit is enlarged and the annular disc pack reversing brake is enlarged at additional expense.
Understandably, a considerable number of alternative planetary transmissions have been employed or suggested which embody different element connections and/or placement of the reversing planetary gear unit and the multiple speed planetary gear units. Exemplifying the art in this area are: U.S. Pat. No. 4,004,473 issued to R. O. Chambers, et al on Jan. 25, 1977; U.S. Pat. No. 4,038,887 issued to N. Murakami et al on Aug. 2, 1977; and U.S. Pat. No. 4,086,827 issued to H. E. Chana on May 2, 1978. However, these alternative transmissions are either relatively uneconomical in construction such as by requiring excessively large components, overrunning clutches, or compound planetary sets, or have other deficiencies such as having relatively high relative speeds between the elements so that the service life of the transmission is adversely affected.
U.S. Pat. No. 4,027,551 issued to N. Murakami et al on June 7, 1977 is still another example of such multispeed planetary transmissions. Undesirably, that transmission utilizes a plurality of concentric tubular connecting elements or shafts that forces the increased diameter of the encircling elements and adds to the complexity of the bearing supports.
Still another problem with prior transmissions is that they do not provide the desired step ratios. Specifically, it is often desirable to have a plurality of relatively close steps or smaller step ratios in the intermediate speed range where the vehicle is more frequently working, and this requirement must be balanced against the need for a wide overall speed range.
The present invention is directed to overcoming one or more of the problems as set forth above.